Diodes and particularly light emitting diodes (LED's) fabricated with conjugated organic polymer layers have attracted attention due to their potential for use in display technology [J. H. Burroughs, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burns, and A. B. Holmes, Nature 347, 539 (1990); D. Braun and A. J. Heeger, Appl. Phys. Lett. 58, 1982 (1991)]. These references as well as all additional articles, patents and patent applications referenced herein are incorporated by reference. Among the promising materials for use as active layers in polymer LED's are poly (phenylene vinylene), (“PPV”), and soluble derivatives of PPV such as, for example, poly(2-methyoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene), (“MEH-PPV”), a semiconducting polymer with an energy gap Eg of ≈2.1 eV: This material is described in more detail in U.S. Pat. No. 5,189,136. Another material described as useful in this application is poly(2,5-bis(cholestanoxy)-1,4-phenylene vinylene), (“BCHA-PPV”), a semiconducting polymer with an energy gap Eg of ≈2.2 eV. This material is described in more detail in U.S. patent application Ser. No. 07/800,555. Other suitable polymers include, for example; OCIC10-PPV; the poly(3-alkylthiophenes) as described by D. Braun, G. Gustafsson, D. McBranch and A. J. Heeger, J. Appl. Phys. 72, 564 (1992) and related derivatives as described by M. Berggren, O. Inganas, G. Gustafsson, J. Rasmusson, M. R. Andersson, T. Hjertberg and O. Wennerstrom; poly(paraphenylene as described by G. Grem, G. Leditzky, B. Ullrich, and G. Leising, Adv. Mater. 4, 36 (1992), and its soluble derivatives as described by Z. Yang, I. Sokolik, F. E. Karasz in Macromolecules, 26, 1188 (1993), polyquinoline as described by I. D. Parker J. Appl. Phys, Appl. Phys. Lett. 65, 1272 (1994). Blends of conjugated semiconducting polymers in non-conjugated host polymers are also useful as the active layers in polymer LED's as described by C. Zhang, H. von Seggern, K. Pakbaz, B. Kraabel, H. -W. Schmidt and A. J. Heeger, Synth. Met., 62, 35 (1994). Also useful are blends comprising two or more conjugated polymers as described by H. Nishino, G. Yu, T-A Chen, R. D. Rieke and A. J. Heeger, Synth. Met., 48, 243 (1995) Generally, materials for use as active layers in polymer LED's include semiconducting conjugated polymers, more specifically semiconducting conjugated polymers which exhibit photoluminescence, and still more specifically semiconducting conjugated polymers which exhibit photoluminescence and which are soluble and processible from solution into uniform thin films.
In the field of organic polymer-based LED's it has been taught in the art to employ a relatively high work function metal as the anode; said high work function anode serving to inject holes into the otherwise filled π-band of the semiconducting, luminescent polymer. Relatively low work function metals are preferred as the cathode material; said low work function cathode serving to inject electrons into the otherwise empty π*-band of the semiconducting, luminescent polymer. The holes injected at the anode and the electrons injected at the cathode recombine radiatively within the active layer and light is emitted. The criteria for suitable electrodes are described in detail by I. D. Parker, J. Appl. Phys, 75, 1656 (1994).
Suitable relatively high work function metals for use as anode materials are transparent conducting thin films of indium/tin-oxide [H. Burroughs, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burns, and A. B. Holmes, Nature 347, 539 (1990); D. Braun and A. J. Heeger, Appl. Phys. Lett. 58, 1982 (1991)]. Alternatively, thin films of conducting polymers such as poly(aniline), (“PANI”) can be used as demonstrated by G. Gustafsson, Y. Cao, G. M. Treacy, F. Klavetter, N. Colaneri, and A. J. Heeger, Nature, 357, 477 (1992), by Y. Yang and A. J. Heeger, Appl. Phys. Lett 64, 1245 (1994) and U.S. patent application Ser. No. 08/205,519, by Y. Yang, E. Westerweele, C. Zhang, P. Smith and A. J. Heeger, J. Appl. Phys. 77, 694 (1995), by J. Gao, A. J. Heeger, J. Y Lee and C. Y Kim, Synth. Met., 82,221 (1996) and by Y. Cao, G. Yu, C Zhang, R. Menon and A. J. Heeger, Appl. Phys. Lett. 70, 3191, (1997). Thin films of indium/tin-oxide and thin films of polyaniline in the conducting emeraldine salt form are preferred because, as transparent electrodes, both enable the emitted light from the LED to radiate from the device in useful levels.
Suitable relatively low work function metals for use as cathode materials are the alkaline earth metals such as calcium, barium, strontium and rare earth metals such as ytterbium. Alloys of low work function metals, such as for example alloys of magnesium in silver and alloys of lithium in aluminum, are also known in prior art (U.S. Pat. Nos. 5,047,687; 5,059,862 and 5,408,109). The thickness of the electron injection cathode layer has ranged from 200-5000 Å as demonstrated in the prior art (U.S. Pat. No. 5,151,629, U.S. Pat. No. 5,247,190, U.S. Pat. No. 5,317,169 and J. Kido, H. Shionoya, K. Nagai, Appl. Phys. Lett., 67(1995)2281). A lower limit of 200-500 Ångstrom units (Å) is required in order to form a continuous film (full coverage) for cathode layer (U.S. Pat. No. 5,512,654; J. C. Scott, J. H. Kaufman, P. J. Brock, R. DiPietro, J. Salem and J. A. Goitia, J. Appl. Phys., 79(1996)2745; I. D. Parker, H. H. Kim, Appl. Phys. Lett., 64(1994)1774). In addition to good coverage, thicker cathode layers were believed to provide self-encapsulation to keep oxygen and water vapor away from the chemically active parts of the device.
Electron-injecting cathodes comprising ultra-thin layer alkaline earth metals, calcium, strontium and barium, have been described for polymer light emitting diodes with high brightness and high efficiency. Compared to conventional cathodes fabricated from the same metals (and other low work function metals) as films with thickness greater than 200 Å, cathodes comprising ultra-thin layer alkaline earth metals with thicknesses less than 100 Å (e.g., 15 Å to 100 Å) provide significant improvements in stability and operating life to polymer light emitting diodes [Y. Cao and G. Yu, U.S. patent application Ser. No. 08/872,657.
Unfortunately, although the use of low work function electrodes is required for efficient injection of electrons from the cathode and for satisfactory device performance, low work function metals such as calcium, barium and strontium are typically unstable and readily react with oxygen and/or water vapor at room temperature and even more vigorously at elevated temperatures.
Despite the improvements in the construction of polymer LED's, a persistent problem has been fast decay of the device efficiency (and light output) during storage and during stress, especially at elevated temperature. Thus, there is a need for methods of encapsulation of such devices, said encapsulation being sufficient to prevent water vapor and oxygen from diffusing into the device and thereby limiting the useful lifetime.